1. Field of the Invention
The present invention relates to a radiation CT apparatus for examining the internal structure or the existence of a defect of various types of industrial products in a non-destructive manner using radiation, for example, X-rays.
2. Description of Related Art
In industrial CT apparatuses, such a structure is generally used that a rotational table, on which an object to be examined is placed between a radiation source and a radiation detector that are provided so as to face each other, is provided so as to rotate around a rotation axis that is perpendicular to the direction of the axis of radiation (direction along the line that connects the radiation source and the radiation detector).
In such a radiation CT apparatus, a so-called CT scan is carried out in such a manner that the rotational table is rotated while the object is irradiated with radiation, and thus, radiation projection data of the object is collected by taking in the output of the radiation detector for each of the minute rotational angles of the rotational table. The data is used for the arithmetic operation of reconstruction so that a tomogram of the object along the sliced surface perpendicular to the rotation axis is constructed. Positional information on the coordinates of the rotation axis of the rotational table projected over the radiation detector is essential for this arithmetic operation of reconstruction.
As a method for finding the coordinates of the rotation axis projected over the radiation detector, a method for using a phantom Jc for calibrating the position of the projected rotation axis as illustrated in FIG. 6 is generally used. This phantom Jc for calibrating the position of the projected rotation axis has such a structure that a wire Jw made of a material having a high absorbance of radiation, such as tungsten, is provided inside a support member Js made of a material that easily transmits radiation, such as acryl. The rotational table is irradiated with radiation from the source while rotating in such a state that this phantom Jc for calibrating the position of the projected rotation axis is placed on the rotational table, and thus, the output from the radiation detector is taken in so that the data on the projected wire Jw is collected. The coordinates of the rotation axis projected over the radiation detector can be found from the sinogram that is gained from the position of the projected pixel sequence along a plane that is perpendicular to the rotation axis included in the data on the projected wire Jw (see Patent Document 1). Whenever the positional relationship between the radiation source, the radiation detector, and the rotational table is changed, the correct values of the coordinates of the projected rotation axis are found according to the above-described method so that a tomogram without any blurring can be gained.
Another technique has been adopted as a method that is a simplified version of the above-described method. The coordinates of the rotation axis projected over the radiation detector were found in advance according to the above-described technique and stored for each of the multiple states when the neutral positional relationship between the radiation source, the radiation detector, and the rotational table were changed so that these multiple positional relationships and the coordinates of the projected rotation axis were used to find the coordinates of the projected rotation axis that correspond to the positional relationship between the radiation source, the radiation detector, and the rotational table when an image is taken through interpolation calculation or extrapolation calculation.